Field of the Invention
The present invention relates to a condenser microphone that generates an electrostatic capacitance change of a condenser microphone unit as a voice output signal using a phase locked loop (PLL).
Description of the Related Art
The condenser microphone has a condenser type microphone unit having a combination of a diaphragm and a fixed electrode, and extracts the electrostatic capacitance change accompanying displacement of the diaphragm caused by a sound wave as the electric signal. Therefore, a direct current (DC) voltage is applied between the diaphragm and the fixed electrode. This DC voltage is called a polarization voltage. As for methods for applying the polarization voltage, a method for applying a voltage from the outside of the microphone unit and a method for giving the polarization voltage by means of an electret material are generally used.
In addition, since the condenser microphone unit has very high impedance between the diaphragm and the fixed electrode, it is configured to obtain the sound signal using an impedance converter that uses an FET or a vacuum tube.
However, the conventional condenser microphone is susceptible to an external electric field and an external magnetic field when converting the electrostatic capacitance change into a voltage. Moreover, since the impedance converter is used, a noise resulting from electric charge leakage intrinsic to the impedance converter is generated, for example, when surrounding humidity is high.
Furthermore, an electret type condenser microphone unit can be small in size in order to be used for a cellular phone, for example. However, since the electret is sensitive to heat, a reflow solder method cannot be applied to this when being mounted on a substrate. Therefore, the electret type condenser microphone unit was mounted separately from chip components, such as resistors and capacitors, which caused an increase of man-hour for the mounting.
In view of this, Japanese Patent No. 5409293 discloses a circuit configuration that converts the electrostatic capacitance change of the microphone unit into the voice output signal using a PLL circuit without using an electret.
FIG. 5 is a principal part of a circuit configuration disclosed in Japanese Patent No. 5409293. In this example, a three-stage RC network including resistors R and capacitors C and a phase shift oscillator containing an inverting amplifier are used. An output signal of this phase shift oscillator is used as a phase synchronization signal of the PLL circuit.
As shown in FIG. 5, a diaphragm 1 that configures the condenser microphone unit is attached to a diaphragm ring 2. A electrically conductive film (not shown) formed on the diaphragm 1 is connected to the ground. Moreover, the fixed electrode confronting the back side of the diaphragm 1 is divided into first to third fixed electrode pieces 3a, 3b, and 3c each of which is fan shaped. Therefore, three electrostatic capacitors C1, C2, and C3 that are formed between the diaphragm 1 and the respective fixed electrode pieces 3a, 3b, and 3c are used as capacitors of the RC network.
Furthermore, fixed resistors R1, R2, and R3 are connected, respectively, between an output terminal of the inverting amplifier 4 and the first fixed electrode piece 3a, between the first fixed electrode piece 3a and the second fixed electrode piece 3b, and between the second fixed electrode piece 3b and the third fixed electrode piece 3c, and the third fixed electrode piece 3c is connected to an input terminal of the inverting amplifier 4. Thus, the three-stage RC network mentioned above is formed.
Then a signal corresponding to an electrostatic capacitance of the condenser microphone unit is supplied to the PLL circuit from the output terminal of the inverting amplifier 4 as a phase synchronization signal.
The PLL circuit includes a phase comparator 5, a charge pump 6, a loop filter 7, and a voltage-controlled oscillator 8. A control signal obtained by the loop filter 7 of this PLL circuit is brought to an output terminal OUT and this signal is used as the audio output signal of the condenser microphone.
According to this, since the electrostatic capacitance change is not converted into a voltage, the condenser microphone is hard to be affected by influences of the surrounding electromagnetic fields, and a noise by the electric charge leakage of the impedance converter does not occur, either. Furthermore, since the polarization voltage by the electret material and the like is not needed, surface mounting by the reflow solder method together with chip components, such as resistors, and capacitors also becomes possible.